Recently, resistance variable memory elements, which include programmable conductor memory elements based on chalcongenide glasses, have been investigated for suitability as semi-volatile and non-volatile random access memory elements.
The mechanism by which the resistance of a chalcogenide glass memory element is changed is not fully understood. In one theory, the conductively-doped dielectric material undergoes a structural change at a certain applied voltage. It is surmised that a conductive dendrite or filament grows under influence of the applied voltage to extend between the electrodes, effectively interconnecting the two electrodes and setting the memory element to a low resistance state. The dendrite is thought to grow through the resistance variable material along a path of least resistance.
The low resistance state will remain intact for days or weeks after the voltage potentials are removed. Such materials can be returned to a high resistance state by applying a reverse voltage potential between the electrodes, the reverse potential having at least the same order of magnitude as was used to write the element to the low resistance state. Again, the highly resistive state is maintained once the voltage potential is removed. This way, such a device can function, for example, as a resistance variable memory element having two resistance states, which can define two logic states.
As noted above, resistance variable materials of particular interest include chalcogenide glasses doped with a conductive substance that will disperse or migrate within the glass. A specific example is germanium-selenide glass (GexSe100−x), doped with silver (Ag). Another example is a germanium-selenide glass (GexSe100−x) which receives silver ions through an adjacent silver-selenide (Ag2Se) layer.
Known memory elements based on silver-containing chalcogenide materials require that pulses of reverse polarity be applied to switch the memory element between the different resistance states. Since it is often inconvenient to supply a negative potential power source to a memory device, it would be desirable to have a resistance variable memory element which switches between resistance memory states using different levels of a positive voltage.